There are a variety of display systems which rely on the change of color for presenting a dynamic and colorful display. In the art of electric "writing" signs, for example, one common technique has been to use control of the sign's electrical power to "write-out" the sign as it is progressively illuminated from one end to the other. However, a disadvantage is that the sign is only one color, and may be only partially visibly illuminated at any one instant.
There are other systems which make use of different colored or differently illuminated liquids for providing a colorful display. For example, there are devices which comprise an illuminable gas tube with a liquid pipe in overlapping relationship and a circulating system for alternately flowing liquids of different colors through the liquid pipe. In these devices, the alternation is effected by multiple pumps and/or valves controlled by complex electro-mechanical mechanisms to effect the alternate flows. In U.S. Pat. No. 5,075,992 there is shown a fluid circulating system for sequentially circulating different colored liquids through a liquid conduit to produce a visual display as a "writing sign" wherein a transparent conduit for carrying liquid is illuminated by luminous tubing encased in coaxial relation therein. The fluid circulating system includes a reservoir tank, a pump, a metering chamber and a dispensing chamber below the metering chamber, ducts for fluidly communicating the different strata of liquids in the reservoir with the metering chamber, and a siphon line and an air breather line which interconnect the metering chamber with the dispensing chamber. The design, however, tends to produce mixing of air with the circulating liquids and some mixing of the liquids themselves which diminishes the contrast of the colored liquids and blurs the line of separation therebetween as they are circulated through the system.